Plasmin and plasminogen were determined monthly over a one year period in samples of bulk and herd milks from Montbeliard cows. Montbeliard milk showed a high content of plasmin and plasminogen in comparison with milk from other breeds. In bulk milk, the plasmin activity reached a minimum in September (O·15 μg/ml milk) and a maximum in June (0·32 μg/ml milk). The annual mean concentration was 0·23 μg/ml milk. The plasminogen content varied around a value of 1·28 μg/ml milk, with a marked decrease in September (0·83 μg/ml milk) and a maximum in October (1·59 μg/ml milk). In herd milk, the minimum plasmin activity occurred in October (0·17 μ/ml milk) and the maximum in spring (O·42 μg/ml milk in May) with an annual mean of 0·30 μg/ml milk. The plasminogen content varied in a similar way, from 0·87 μg/ml milk to 1·82 μg/ml milk, with an annual mean of 1·46 μg/ml. The ratio plasminogen: plasmin ranged from 1·4 to 9·2 with an average of 4·9. From early to late lactation, plasmin and plasminogen concentrations increased from 0·25 to 0·38 μg/ml milk and from 1·07 to 2·01 μg/ml milk respectively and the plasminogen: plasmin ratio increased from 4·5 to 5·3. Studies of milk from cows at similar stage of lactation within a single herd have shown the necessity for distinguishing between two phases in the early stage of lactation. The first, a very early period, is usually present up to one month and the second occurs in the second and third months. Milk samples with the highest proportion of γ-caseins were not those with the greatest plasmin content but were those with high plasminogen contents, which had increased suddenly from the levels of the preceding month. This suggests a role for the plasminogen activator and inhibitor system.

Associations between BoLA type and milk production were investigated using information from 80 Icelandic cattle from four dairy farms in Iceland. The specificity w6 reduced milk, fat and protein yield. Although w6 has been shown to be associated with increased susceptibility to mastitis, there is no evidence to suggest that the reduced yields here were due to mastitis. The association may be due to increased disease susceptibility or due to a direct effect of BoLA Class I antigens.

Starting in week 5 of lactation, goats were milked unilaterally once daily for 4 ½ weeks, then twice daily for 12 weeks and finally thrice daily for 2 weeks. The control gland was milked twice daily throughout. During less frequent milking, yield fell by 26%. There was a rapid and essentially complete recovery of milk yield on resumption of twice-daily milking, and five of the six goats subsequently responded to thrice-daily milking with a small increase in milk yield. Mammary function was assessed during each phase by determining enzyme activities and in vitro synthesis rates of lactose and casein in mammary biopsies. There was a trend towards reduced differentiation of secretory cells during infrequent milking, with very little recovery during twice-daily milking but complete recovery during thrice-daily milking. The differentiative state of the control gland remained unchanged throughout.

The yields and composition of milk from nursing mares were studied during the first two months of lactation in 11 mares of heavy breeds (784 kg). Daily yield increased from 21·7 to 24·6 kg between weeks 1 and 8 of lactation. Fat, protein, gross energy and Ca concentrations significantly decreased when lactose content increased during this period. Individual variations were higher for yield than for composition. Casein, whey protein and non-protein N (56, 34 and 10% of crude protein, respectively) and amino acid composition did not vary between weeks 1 and 8 of lactation.

Responses of dairy cows given silage diets to the intraruminal infusion of urea in progressively increasing doses were studied in four experiments, two with non-lactating cows and two with lactating cows. No clinical symptoms of NH3 toxicity were observed in any of the experiments. When urea was infused continuously, silage intake was depressed (P < 0·05) when the total supply of N exceeded the equivalent of 250g crude protein (CP)/kg DM in the total diet. However, when the urea load was administered twice daily, as opposed to continuously, intake depression (P < 0·05) occurred at the equivalent of 170g CP/kg DM. At the higher doses of urea, concentrations of NH3 in peripheral blood increased and were accompanied by increased concentrations of glucose and reduced levels of insulin in plasma. In general, responses of milk production followed those of silage intake but there was evidence of greater proportional reductions in the yield of lactose relative to that of fat and protein. It is concluded that the voluntary intake of high-protein silages may be depressed by factors associated with high rates of absorption of NH3 from the rumen.

The hyperinsulinaemic euglycaemic insulin clamp technique was used to compare insulin sensitivity in lactating ewes at two levels of feeding. Clamps were performed at two (restricted intake) or three (ad libitum intake) stages of lactation and also 30 d after drying off. Dose response curves for insulin were constructed using the glucose metabolic clearance rate (MCR) as the measure of glucose metabolism and these were statistically compared between stages of lactation within the feeding levels, and also between feeding regimes. Animals on a restricted feed intake showed a weight loss throughout lactation, coupled with a lower insulin sensitivity, as measured by the ED50 (concentration of insulin required to produce a half maximal increase in MCR), while those on ad libitum feeding showed a weight gain and a decreased insulin sensitivity in the dry period. Endogenous glucose production was less sensitive to inhibition by insulin in the animals subjected to restricted food intake, compared with those on the ad libitum regime, which suggested a major role for the plane of nutrition in adjusting the homeorhetic control of metabolism during lactation.

Lactating goats were subjected to dehydration and to infusions of arginine vasopressin (AVP). Dehydration decreased milk production significantly but no changes were observed on the day of AVP infusion. The levels of AVP in plasma reached the same magnitude during both experimental procedures. AVP-immunoreactivity was immunocytochemically detected by light (peroxidase-antiperoxidase technique) and electron microscopy (immuno-Au technique on to glutaraldehyde-fixed, resin-embedded sections) in the mammary gland but not in the control experiments. In addition the cisternae of the rough endoplasmic reticulum in the secretory alveolar cells underwent significant swelling in response to the experimental procedures.

In five untreated goats, mammary extraction of plasma triglycerides increased at parturition. In five goats that had as much secretion as possible regularly removed from one mammary gland beginning on d 143 of gestation, the change of triglyceride extraction occurred several d before parturition in the emptied gland but at parturition in the contralateral control gland. The experimentally induced early increase of extraction of plasma triglycerides preceded the increase of prostaglandin Fα concentration in mammary secretion at parturition, and occurred despite abnormally high concentrations of progesterone in the secretion.

Machine milking induced repeatable, short-term changes in thickness of the teat apex. These changes were measured with a spring-loaded caliper instrument under milking conditions that imposed differing forces on the teat apex. The mean percentage changes in teat end thickness (relative to the premilking values for individual teats) varied from 10% decrease up to 20% or more increase depending on the particular milking system used. The overall decrease in teat end thickness associated with cannula milking, milking at very low vacuum levels or milking with positive pressure pulsation may result from: (i) decrease in the intramammary pressure; and/or (ii) small changes in tone of teat musculature; and/or (iii) changes in the distribution of interstitial fluids in the teat apex. The overall increase in teat thickness caused by conventional or PKME milking systems results mainly from oedema (the extravascular accumulation of fluid). It is likely that milking equipment can, and should, be designed to minimize circulatory impairment resulting in oedema of the teat.

Affinity chromatography on a concanavalin A–Sepharose support was used to isolate two glycoprotein fractions from a heat- and acid-stable fraction of ovine milk. One of these glycoprotein fractions was purified by rechromatography on DEAE-cellulose to essentially a pure protein yielding a single band on gel electro-phoresis. The apparent Mr of this glycoprotein (GP2) as estimated by electrophoresis was 50500. It contained 8·88% carbohydrate and 0·61% P. The other glycoprotein fraction (GP3) contained 0·53% P and 17·76% carbohydrate including sialic acid, mannose, galactose, fucose, galactosamine and glucosamine. It appeared on electrophoresis in acrylamide gels as a slow-moving broad band. On similar treatment in the presence of sodium dodecyl sulphate it revealed four glycoprotein zones with apparent Mr of 15200, 18300, 23500 and 25300. Both GP2 and GP3 contained low amounts of aromatic and sulphur-containing amino acid residues and large amounts of Asp, Glu, Ser and Leu. GP3 is similar in some respects to the bovine milk heat- and acid-stable fraction constituent, component 3.

A total of 116 molecular species of triglycerides were identified in milk fat, using a combination of HPLC and GLC. Triglyceride composition was predicted from the random composition, which was calculated on the basis of the mole fractions of the main fatty acids making up the total triglyceride fraction. The qualitative composition of the milk fat was similar in cows', ewes' and goats' milk. In all three milks the partition number of the main triglycerides was 46, but the proportions of the triglycerides with partition numbers of 34, 38, 42, and 48 exhibited substantial differences among the milks of the three species.

Acid casein was prepared under various conditions of pH, concentration and wash regimens (temperature, time, agitation and wash water ratio) according to a response surface experimental design. The resulting product was analysed for water-holding capacity and loss of fines, and the structure of the curd assessed by scanning electron microscopy. Concentration, pH and temperature were the major factors in terms of influencing fines, water-holding capacity and the microstructure of the curd. Fines could be minimized by selecting appropriate pH, concentration and agitation conditions. The final moisture content of the curd was mainly controlled by pH, concentration and temperature and may aid in the selection of parameters to obtain good dewatcring and drying efficiencies. Microstructural analysis visualized the effects of pH, concentration, temperature and their interactions on the casein structural network and the effects of the process parameters are discussed in relation to those changes. Response surface methodology was shown to be an effective tool for examining which variables used in the process were significant and provided guidance as to the type of physicochemical mechanisms affecting the system.

Effect of heating and of some chemical treatments on the proteolysis of skim milk caused by its major proteinase system, plasmin, was investigated. Degree of proteolysis was expressed as the increase in γ-casein content of skim milk (mg/ml) after incubation with 0·02% (w/v) NaN3 at 37 °C for 20 h. Apart from reduced proteolysis in skim milk heated at > 70 °C for 10 min or at 75 °C for ∼ 5 min, some enhancement (10–80%) was observed on heating at 55–63 °C for 10 min or at 60–65 °C for 15 s. Heating at pasteurization conditions (63 °C/30 min or 72 °C/15 s) had no apparent effect on the amount of proteolysis. Addition of ascorbic acid (0·2 mg/100 ml) or H2O2 (1/100 ml) to unheated skim milk also increased the amount of proteolysis by 38 and 28%, respectively. However, the extent of this increase diminished with increasing amounts of both chemicals. Furthermore, there was a steady increase (∼ 33%) in proteolysis with time of exposure to light for up to 3 h. These results suggest that oxidative conditions lead skim milk to enhanced proteolysis by inactivating an inhibitor(s) operating in the plasmin system in milk.

Properties of interest to cheesemaking were investigated with milks concentrated by ultrafiltration and heated at 90 °C or 95–97 °C for 15 s. The effects of light homogenization before concentration and of addition of CaCl2 after heating were assessed. No changes in casein–fat or casein–whey protein interactions were detected by electron microscopy. The heat denaturation of whey protein increased linearly as the milk became more concentrated. Coagulability by rennet after heat treatment increased with concentration to close to the unheated value in 3·5 to 4-fold concentrates. The decrease in curd firming rate by heat treatment was slightly affected by concentration and addition of CaCl2 restored some of the difference. Heat treatment reduced the rate of whey loss and slightly improved the curd structure but did not affect fat losses. Light homogenization slightly reduced heat denaturation of whey protein and whey loss.

Cheddar cheeses were made from milks concentrated approximately 2·3-and 3·4-fold by ultrafiltration, then heated at 90 °C for 15 s. They were compared with cheeses made from pasteurized unconcentrated milks and unheated 5·3-fold concentrates. The recoveries of fat and protein were comparable in the cheeses from heated 2·3-fold concentrates and unheated 5·3-fold concentrates and both were higher than the controls. However, fat losses were high in curds from 3·4-fold heated concentrates. The use of paired fast and slow starter strains and coagulation at 26 °C optimized the curd composition. Proteolysis and flavour development were only slightly reduced in all the cheeses from concentrated milks, perhaps aided by higher starter levels in the curd than in the controls. The structures of the curds from the heated concentrated milks were coarser than those of the controls and the curd particles fused poorly. This may explain the grainy, dry, crumbly texture in the cheeses from the heated concentrates. The texture was not improved by adding a bacterial proteinase to the curd.

Partly purified extracellular aminopeptidase from Brevibacterium linens was used to accelerate Cheddar cheese ripening. It was found that although the aminopeptidase was unstable in acidic buffer, it was highly stable in Cheddar cheese; negligible amounts of the enzyme activity were lost during 3 months' maturation. A better score for flavour in sensory analysis of enzyme-treated cheese was obtained by the combination of the aminopeptidase and a commercially available metalloproteinase (Neutrase) than by the metalloproteinase alone.

The interactive effect of two of the major Cheddar cheese flavour compounds, H2S and methanethiol, was studied in milk-fat-coated microcapsules containing Brevibacterium linens, cysteine and methionine. Two substrates, cysteine and methionine, were either co-encapsulated with B. linens or encapsulated separately with B. linens and mixed together. The production of H2S from cysteine in microcapsules was slightly reduced by the methanethiol produced from methionine. However, the presence of H2S stabilized methanethiol produced from methionine in milk-fat-coated microcapsules and subsequently increased the level of methanethiol. Without H2S, the methanethiol in milk-fat-coated microcapsules disappeared rapidly after 48 h. By making separate microcapsules at each optimum environment and mixing together, the cells could operate in the protected and ideal environment of the microcapsules to form products that diffused out and acted as reactants with products from other microcapsules.

The water activity of fresh cheese made from cows', ewes' and goats' milk, as well as curd cheese and whey cheese, responds to the equation aw = 0·9719−0·0044[NaCl] + 0·0041 pH, with a square of multiple correlation coefficient R2 = 0·95. This equation was obtained by multiple regression analysis of data point sets of the variables x (g NaCl/100 g moisture), y (pH) and z (water activity, aw) determined as the average of values provided by six samples of each unripe product. Application of this equation to 30 individual samples of unripe dairy products investigated yielded calculated aw values within ±0·005 and ±0·002 aw units of the experimental water activities in 86·7% and 53·3% of the samples, respectively. These results are closer to the experimental values than those provided by earlier predictive equations.

Proteinase, peptidase and esterase activities were detected in cell-free extracts of four Lactobacillus delbrueckii subsp. bulgaricus and four Streptococcus salivarius subsp. thermophilus strains. Post-electrophoretic detection was based on hydrolysis of L-leucine-βnaphthylamide and α-naphthylacetate. The substrates L-leucine-p–nitroanilide, N-acetyl-L-alanine-p–nitroanilide, 2-nitrophenylbutyrate and 4-nitrophenylbutyrate were used for the spectrophotometric detection of the enzymes. Estimation of total proteolytic activity was based on hydrolysis of bovine whole casein. Interesting differences were observed between the two French strains and those isolated from traditional Greek yogurt.